Oxyapatite laser materials

ABSTRACT

A COMPOSITION OF MATTER WHICH CAN BE USED AS A LASER CRYSTAL AND WHICH CAN BE DOPED WITH SENSITIZER IONS HAS THE EMPIRICAL CHEMICAL FORMULA RM4-X(ZO4)3O:AX WHERE A REPRESENTS A LASING ION SELECTED FROM ND, ER, AND HO, X HAS A VALUE FROM .001 TO 1, M IS AN ION SELECTED FROM LA, GD, Y AND MIXTURES THEREOF, Z IS AN ION SELECTED FROM SI AND GE AND R IS AN ION SELECTED FROM SR, BA, AND MG.

Jan. 11, 1972 w. ROLAND E AL 3,634,281

OXYAPATITE LASER MATERIALS- 2 Sheets-Sheet 1 Filed Sept. 22, 1969 GROUND STATE ACTIVATOR SENS'ITIZER FIG.

. .nv M U mm a C E nm P m S S CLIP C I d N S C W 5.0 R M 06 U S M L F H T G N 1 0 E W A W 1 L P 7.

Jan, 11,1972 RO AND ETAL 3,634,281

OXYAPATITE LASER MATERIALS Filed Sept. 22. 1969 2 Sheets-Sheet 2 0.6 WAVELENGTH, MICRONS l l l l l I l 1 I l q SllNfl MIVHLIGHV WfllNVflO NOllVllDXi 33d NOISSIW3 United States Patent flice 3,634,281 OXYAPATITE LASER MATERIALS George W. Roland and Richard H. Hopkins, Monroeville, and Nathan T. Melamed, Pittsburgh, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed Sept. 22, 1969, Ser. No. 859,753

Int. Cl. (109k 1/54 US. Cl. 252-301.4 F 12 Claims ABSTRACT OF THE DISCLOSURE A composition of matter which can be used as a laser crystal and which can be doped with sensitizer ions has the empirical chemical formula RIVI (ZO O:A where A represents a lasing ion selected from Nd, Er, and Ho, x has a value from .001 to l, M is an ion selected from La, Gd, Y and mixtures thereof,Z is an ion selected from Si and Ge and R is an ion selected from Sr, Ba, and Mg.

BACKGROUND OF THE INVENTION Energy transfer from one fluorescent specie to another or among fluorescent species of the same kind, is a fundamental process in luminescence. Before the advent of lasers, energy transfer was widely utilized in commercial phosphors such as those used in fluorescent lamps to improve their efliciency, and was extensively studied in connection with organic phosphors.

With the advent of lasers, energy transfer processes have taken on additional importance as a means for improving the efliciency of optically pumped lasers. The work on fluorescent lamps was concerned mainly with the transfer of energy between transition metal ions of different types. In contrast, investigations on laser materials have been principally concerned with energy transfer from transition metal ions to rare-earth ions, or energy transfer from rare-earth to rare-earth ions.

The basic aim of laser energy transfer can be described as follows: given an ion which has desirable spectroscopic properties (i.e., it emits in a desirable frequency region with a suitable bandwidth etc.), but which is only a weak or ineflicient absorber of the excitation energy, one must find another ion which has desirable absorption properties and which can transfer its energy efliciently and rapidly to the emitting ion. The emitting ion is called the activator or lasing ion and the absorbing ion is called the sensitizer. Energy transfer occurs from the sensitizer to the activator ion.

It has been demonstrated in US. Ser. No. 732,593, filed on May 28, 1968, and assigned to the assignee of this invention, that the mineral fluorapatite Ca (PO )F, is an excellent host for sensitizer and/ or activator ions. Suitably doped fluorapatite exhibits high gain and low threshold characteristics. Large single crystals of this doped material are prepared by Czochralski growth from stoichiometric melts at temperatures of about 1650 C.

Our invention relates to a composition of matter suitable as alaser crystal in a resonant cavity of a laser generator. Our laser materials are based on silicate or germinate oxyapatite hosts doped with neodymium, erbium or holmium. Within the limits of our measurements, these materials melt congruently at considerably higher temperatures (about 2000 to 2200 C.) than fluorapatite. Although the existence and synthesis of some oxyapatite powders generally has been disclosed, as for example by Jun Ito in 53 American Mineralogist 890; the growth, doping and laser application of large single crystals of our materials has not been previously considered.

In addition to the crystalline laser materials of this invention, other crystalline laser materials are described 3,6342% Patented Jan. 11, 1972 in US. patent applications Ser. Nos. 859,673, 859,672 and 859,754, all ifiled on Sept. 22, 1969 and assigned to the assignee of this application.

SUMMARY OF THE INVENTION It is the prime object of this invention to provide a new and improved high strength composition of matter for use as a laser crystal in the resonant cavity of a laser generator.

This invention accomplishes the foregoing object by providing a silicate or germinate oxyapatite laser crystalline material having the empirical formula:

where M represents an ion selected from La, Gd, Y and mixtures thereof. R and M are considered host constituents. This is because they are not activators and play no role as sensitizer ions. M, which can be La, Gd and Y ions, is necessary to the construction of the host crystal lattice and is the prime constituent for which activator and sensitizer ions are substituted. A represents an activator ion (lasing ion) that is reponsible for laser output. A is selected from Nd, Er or Ho. The ion which is the lasing ion, A, in the crystal can be determined by measuring the frequency of the laser oscillations and from known spectroscopic data. Generally only one lasing ion will oscillate at a time. S, which may be optionally present, represents a sensitizer ion. The sensitizer ion must be matched to the lasing ion. The value x can vary between .001 to 1 with a preferred range between .001 to .3 and y can vary between 0 to (4-x) with a preferred value between 0 to 1. Z is an ion selected from Si and Ge and R is an ion selected from Sr, Ba, and Mg.

Our laser materials have low threshold characteristics and reasonably low gain allowing improved energy storage. They also have high material strength associated with high-melting-point compounds. Our materials provide a laser crystal capable of withstanding, without structural damage significantly higher pumping energies than fluorapatite. Little or no segregation is observed between melt and crystal capable of withstanding, without structural damage significantly higher pumping energies than fluorapatite. Little or no segregation is observed between melt and crystal using neodymium as an activator. This reduces serious crystal homogeneity problems which occur with most hosts due to variation in dropant concentration along the crystal caused by temperature fluctuations during growth. Segregation should also be minimal with other dopants such as Er and Ho.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the nature and objects of the invention, reference may be made to the following drawings in which:

FIG. 1 shows energy levels of sensitizer and activator ions indicating transitions pertinent to energy transfer;

FIG. 2 shows a laser generator utilizing the laser crystal of this invention in association with a radiation source in a resonant laser cavity;

FIG. 3 shows the fluorescence spectrum of a polycrystalline sample of SrLa Nd (SiO O; and

FIG. 4 shows the excitation spectrum of the infrared fluorescence from a polycrystalline sample of DESCRIPTION OF THE PREFERRED EMBODIMENTS The oxyapatite host materials of this invention have the formula RM (ZO O where M is a host constituent ion selected from La+ Gd+ Y+ and mixtures thereof, Z is an ion selected from Si+ and Ge and R is a host constituent ion selected from Sr+ Ba+ and Mg. These hosts contain ion sites which will accommodate both rare earth and transition metal ions. The crystals structure of the host materials is hexagonal with a unit cell formula of R M (ZO O These host materials have an apatite structure (space group 26 /111). The Si+ or Ge+ ions are in SiO or GeO tetrahedra. Two sets of ions are present; one set is co-ordinated with Si+ or Ge+ in the $0., or GeO tetrahedra and the other set occurs along the c axis (two ions per unit cell) with each ion coordinated by three cations (R or M ions) in the plane of the horizontal mirror (in 26 /111). Two types of cation sites are present (R or M and R or M The activator ions and/or transition metal or rare earth sensitizer ions can substitute for M and R in the host materials. This will be a substitution of some of the five R and M cations in the host having the formula RM (Z00 0 (where M represents an ion selected from La, Gd, Y and mixtures thereof and R represents an ion selected from Sr, Be and Mg).

The host materials of this invention use the rare earth ions Nd+ Er+ or Ho+ as their activator (lasing) ion, A. The ion concentration of these activators can vary in the host from about .02 to 20 atom percent of the five cations (one R and four M cations) in the host, RM (ZO O. The preferred range is from about .02 to 6 atom percent. Below the preferred range there is generally not enough optical absorption and above the preferred range there may be concentration quenching. Thus, x in the formula RM (ZO O:A has a preferred value between .001 and .3, i.e., (001=x)/5 cations equals .020 atom percent and (.3=x)/5 cations equals 6 atom percent. However with improved flash sources for special application it is useful to have the value of x greater than .3. The sensitizer ion S has a preferred range for y from 0 to 1 i.e. the ion concentration of S preferably varies from about 0 to 20 atom percent (1=y/5) of the five cations in the host.

In accordance with this invention, sensitizer ions may be used to sensitize the rare earth activator ions Nd+ Er+ or Ho+ in the host. Referring now to FIG. 1 which illustrates the various steps involved in non-radiative energy transfer: (1) the sensitizer ion absorbs a photon of external radiation, of energy r, lifting it from the sensitizer ground state D to an excited state A; (2) the sensitizer subsequently decays to a lower metastable state N, by the emission of a photon r or by a non-radiative process; (3) once lattice relaxation about the sensitizer metastable state has taken place, the sensitizer is either free to radiate a photon r" or to transfer its energy to an activator ion, as indicated by R; (4) if the electronic transitions in both the sensitizer and activator are electric dipole transitions, the dipole field of the excited sensitizer can induce a dipole transition in a nearby activator, thereby raising the activator to an excited state A, with a simultaneous return of the sensitizer to its ground state; (5) this transition transfers a quantum of energy from the sensitizer to the activator. Once excited, the activator can decay to a lower metastable level N, through emission of photons, and can eventually decay to its ground state D either directly or via an intermediate level D". Reference may be made to D. L. Dexter, J. Shem. Phys, vol. 21, 1953, page 836 for detailed descriptions of these energy transfer processes.

The requirements in non-radiative transfers for efficient transfer of energy from sensitizer to activator are: (1) a reasonable overlap in energy between the sensitizer emission band and an absorption band of the activator; (2) high oscillator strengths to both sensitizer and activator; (3) a relatively high intrinsic radiative quantum efiiciency for both the sensitizer and activator. In addition to the above criteria, there are several other criteria of a more general nature for a useful sensitizer. These are: (l) the sensitizer should absorb radiation in a spectral region where the activator has little or no absorption; (2) the sensitizer should absorb in a region where the pump lamp radiates appreciable energy, and (3) the sensitizer should not absorb where the activator emits, or have any adverse effects on the radiative efliciency of the activator. For efficient energy transfer to occur it is necessary that the rate of transfer (R in FIG. 1) be more rapid than the rate of decay of the sensitizer to its ground state (r in FIG. 1).

The sensitizing ions that may be used in the composition of this invention would include transition metal and rare earth ions which are capable of (a) absorbing radiation energy otherwise not absorbed by the activator (lasing ion) and (b) transferring that absorbed energy to the activator.

Although a variety of transition metal ions and rare earth ions are suitable to sensitize the activator ions in the host laser crystal, best results are achieved when selected sensitizer ions are matched to activator ions in the particular host. The following table shows some suitable combinations:

In the preparation of the laser crystalline material of this invention 26.4382 grams of SrCO 113.7801 grams of La O 3.0132 grams of Nd O and 36.7684 grams of silicic acid were mixed together. All reactants were of luminescent grade (greater than 99.9% purity). The ingredients were then placed in an iridium crucible and melted at approximately 2170 C. as measured by an uncorrected optical pyrometer.

Crystals were pulled from the melt at 2180 C. using the standard Czochralski technique, well known in the art and described in an article by J. Czochralski in Zeitschrift fur Physikalische Chemie, vol. 92, pages 219- 221 (1918). A recent description of the process is found in an article by K. Naussau and L. G. Van Uitert in Journal of Applied Physics, vol. 31, page 1508 (1960).

The furnace was surrounded by a quartz cylinder attached to the apparatus by means of a neoprene gasket and a brass flange. Insulation for the iridium crucible was provided by encasing the crucible in zirconia powder contained in a zirconia tube with a A inch thick wall. Thermal distribution throughout the melt was controlled by adjusting the crucible arrangement in the field of the work coil and modifying the height of the zirconia top plate. The power source was a Westinghouse 30 KVA motor-driven 10,000 cycle generator driving a water cooled copper work coil. The pulling apparatus was designed such that pull rates between 1 and 40 mm./hr. and rotation speeds of 10-170 r.p.m. could be used. Temperature was controlled by using the output of a sapphire light pipe leading to a radiamatic detector which fed the output into an L and N Azar recorder-controller. The voltage from the recorder-controller is association with an L and N current adjusting type relay supplies the input current of a Norbatrol linear power controller. The Norbatrol output voltage supplies the necessary field excitation required by the 10,000 cycle generator.

The seed was held on a water cooled shaft which was threaded to accommodate an iridium chuck. The crucible and chuck were protected from oxidation by an argon atmosphere. Oriented seeds were used for growth. These were obtained by starting with a polycrystalline seed obtained from a slow-cooled melt. Crystals were grown as large as 7 inch in diameter and inch long. Cooling rates of the pulled crystals varied from two to three hours.

The crystalline materials that were grown are useful as laser crystal rods in simple lasers and in more complicated laser applications such as Q-switched lasers, both of which are described in detail in chapters 3 and 4 and especially pages 132160 of The Laser by W. V. Smith and P. P. Sorokin, McGr-aW-Hill, 1966, herein incorporated by reference.

A simple schematic illustration of a typical laser generator is shown in FIG. 2 of the drawings. Between reflectros 20 and 21 there is a resonant laser cavity containing the laser crystal 22, a radiation source means 23 such as a flash lamp which provides pump energy to the crystal, and an optional Q-switching means by dotted lines. Reflector 20 is partially reflecting to permit the escape of light beams of coherent radiation 24 whereas reflector 21 is highly reflective.

The basic principle involved in Q-switching a laser is to allow a very high population inversion to be built up by making the laser cavity losses excessive, while the laser is being pumped, thereby preventing the laser from oscillating prematurely. When a strong inversion is attained, the conditions are suddenly made favorable for oscillation by rapidly making the cavity losses very small, so that a condition of large net amplification is suddenly realized. The Q-switch could, for example, contain a metallic-organic compound in solution such as a phthalocyanine which absorbs light from the crystal. The pumping energy input from the flash lamp increases until amplification in the laser crystal overcomes the loss due to absorption in the Q-switch cell and the laser begins to emit coherent light weakly. A very small amount of the light bleaches the solution which then becomes almost perfectly transparent to the light. At that instant there is suddenly a giant pulse of light containing all the stored energy in the laser rod.

One of the crystals pulled at a rate of 0.17 inch per hour from a melt at about 2180 C. showed laser action at a 1.06 micron wavelength. The crystal composition was SrLa Nd (SiO O. This grown boule was ground and polished. The finishing procedure on the rod end resulted in polished ends parallel to better than 6 are seconds and plane to A wavelength of He light. It was in the form of a 0.75" x 0.18" diameter rod. It was tested in a 2.7" diameter cylindrical reflecting cavity with flat reflectors The laser head used in all pulse tests was a cylindrical pyrex reflecting cylinder 75 mm. in diameter and 76 mm. long having two reflecting pyrex end plates with holes machined for the lamp and rod. Front surface evaporated aluminum coatings were used and overcoated with M10 quartz for protection. Resonator reflectivities were both 99.2%. A PEK Xel-3 xenon flashlamp was located diametrically opposite the laser rod with a center to center spacing between the lamp and rod of 0.6 inch. This flashlamp was a broad band emitter with a peak emission around 5800 A. The laser rod was supported in a doublewall pyrex cylinder filled with a water filter solution of NaNo to prevent UV from reaching the laser rod being tested. The flashlamp was powered by a charge 340 ,uf. capactior which was discharged through a 1.50 ,uh. inductor in series with the lamp. The maximum energy into the flashlamp was held below 100 joules to insure long life. The RLC circuit described produced a flashlamp pulse duration of about 800 ,useconds.

Although the operating characteristics of lasers are determined by the properties of their active ions, the actual results achieved in any given system is highly dependent on imperfections in the crystal. Microscopic imperfections invisible to the eye may make laser oscillations impractical. The presence of bubbles of inclusions may scatter the beam and increase threshold significantly.

Despite the small rod size of the SrLa Nd (SiO O crystal and its poor optical quality, room temperature laser action was obtained at the surprisingly low threshold of approximately 12.8 joules. With larger, high-quality crystals, much lower threshold values should be possible.

Measurements of the laser threshold were accomplished by aligning the laser rod in the laser head in the usual way with external reflectors. A 1P25 phototube was then placed in the path of the beam with a 1.06 micron interference filter between the laser and phototube to reduce the background signal. The phototube output was displayed on a Tektronix type 555 dual-beam oscilloscope with one trace serving as an expanded scale. Thresholds could be accurately determined since the onset of laser action appears as characteristic spikes as seen with other materials such as ruby.

Spectroscope data on the fluorescence and excitation of SrLa Nd (SiO O are shown in FIGS. 3 and 4. The fluorescence spectrum (FIG. 3), from the above crystal shows in the near infrared including the 1.06 micron emission corresponding to the Nd laser line. In SrLa Nd (SiO O this line is about 12 times broader than in neodymium doped calcium fluorophosphate (75 A. vs. 6.5 A). Hence, our new silicate oxyapatite host, doped with neodymium should have enhanced saturation-energystorage capabilities making it very promising for Q- switching laser applications.

The excitation spectra (FIG. 4) of the 1.06 emission line of Nd+ show that the energy is transferred from the absorbing levels to the 1 state, the initial laser level.

The melting point of neodymium doped SrLa (SiO O is significantly higher than neodymium doped Ca (PO )F. The hardness of neodymium doped SrLa (SiO O' is also high, and all data indicates that SrLa (SiO O:Nd should be superior to doped calcium fluorophosphate in resisting structural damage and failure at high pump levels.

The excitation and fluorescence spectrometer system consisted of two grating monochromators for dispersing the exciting light and the fluorescence light, along with associated optics, detectors, lamps and electronics. The source used was an Osram Type XBO-900, a high pressure xenon arc lamp which was operated from a DC supply having less than 1% ripple. Fluorescence measurements were made using a Jarrell-Ash monochromator. A 600 1/ mm. grating blazed at 4000 A allowed excitation spectra to be taken from 2500 to 10,000 A. The quantum detectors used RCA 7102 photomultipliers cooled to liquid N temperature.

We claim as our invention:

1. A composition of matter comprising an oxyapatite host having the formula RM (ZO O wherein R is an ion selected from the group consisting of Ba, Sr, and Mg, M is an ion selected from the group consisting of La, Gd, Y and mixtures thereof, and Z is an ion selected from the group consisting of Si and Ge, said host containing an activator ion selected from the group consisting of Nd, Er, and H0 in the ion concentration range of .02 to 20 atom percent of the R and M cations in the host.

2. The composition of claim 1 wherein Z is Si.

3. The composition of claim 2 wherein the host contains the activator ion Nd in the ion concentration range of .02 to 6 atom percent.

4. An oxyapatite laser crystal having the empirical formula RM (ZO O:A S wherein R is an ion selected from the group consisting of Ba, Sr and Mg, M is an ion selected from the group consisting of La, Gd and Y, Z is the ion Si, A is the ion Er, S is the sensitizer ion Yb, x has a value between 0.001 and l, and has a value between 0 and (4-x).

5. The laser crystal of claim 4, wherein y' has a value between 0 to 1 and x has a value between 0.001 and 0.30.

6. The laser crystal of claim 5 wherein y=0.

7. An oxyapatite laser crystal having the empirical formula RM (ZO O:A S wherein R is an ion selected from the group consisting of Ba, Sr and Mg, M is an ion selected from the group consisting of La, Gd and Y, Z is the ion Si, A is the rare earth ion Ho, S is a sensitizer ion selected from the group consisting of Cr,

7 Tm, Br and Yb, x has a value between 0.001 and 1 and y has a value between 0 and (4-2:).

8. The laser crystal of claim 7, wherein y has a value between 0 to 1 and x has a value between 0.001 and 0.30.

9. The laser crystal of claim 8, wherein y=0.

10. An oxyapatite laser crystal having the empirical formula RM (ZO O:A, S, where R is an ion selected from the group consisting of Ba, Sr, and Mg, M is an ion selected from the group consisting of La, Gd and Y, Z is the ion Si, A is the ion Nd, S is the sensitizer ion Mn, wherein A is present in the ion concentration range of 0.02 to 20 atom percent of the R and M cations in the formula and S is present in the ion concentration range of 0 to 20 atom percent of the R and M cations in the formula.

11. The laser crystal of claim 10 wherein R is Sr and M is La.

12. The laser crystal of claim 10 wherein the ion concentration of 5:0.

References Cited Ito: Silicate Apatites and Oxyapatites, 53, American Mineralogist, pp. 894 and 895, June 1968.

ROBERT D. EDMONDS, Primary Examiner 

